The silicon based MEMS microphones extensively used in hearing aids, mobile phones, digital cameras and toys are being pushed to their limits. The need is endless for a smaller, cheaper, robust microphone with a high sensitivity and a low noise level. Reducing the size of the microphone may deteriorate its performance, such as sensitivity. However, nowadays, micro-processing technologies provide a very good control over crucial dimensions and properties of the microphones, making further miniaturization and optimization of the microphone possible. Also, more complicated MEMS microphone principles can be applied, such as those with differential condensers and differential preamplifiers, allowing the performance of the MEMS microphone such as sensitivity to be maintained or improved.
In a non-patent document (Jesper Bay, Ole Hansen, and Siebe Bouwstra, Design of a silicon microphone with differential read-out of a sealed double parallel-plate capacitor, The 8th International Conference on Solid-State Sensors and Actuators, and Eurosensors IX, Stockholm, Sweden, Jun. 25-29, 1995), a sealed capacitive microphone with differential capacitive read-out and high sensitivity is provided, wherein the microphone consists of two diaphragms with a perforated center electrode in between, and the two diaphragms may be interconnected with pillars. This double diaphragm structure is disadvantageous in that it gives common mode change in capacitance, and the diaphragms are less compliant in case pillars are provided therebetween.
In another non-patent document (P. Rombach, M. Mullenborn, U. Klein and K. Rasmussen, The first low voltage, low noise differential condenser silicon microphone, The 14th European Conference on Solid-State Transducers, Aug. 27-30, 2000, Copenhagen, Denmark), a low voltage, low noise differential condenser silicon microphone is presented, which includes a non-planar structure and has all structural layers deposited consecutively and etched back separately to form the structure. However, this structure may not be suitable for integration with CMOS circuitry and/or flip-chip bonding.
In still another non-patent document (David T. Martin, Jian Liu, Karthik Kadirvel, Robert M. Fox, Mark Sheplak, and Toshikazu Nishida, A micromachined dual-backplate capacitive microphone for aeroacoustic measurements, Journal of Microelectromechanical Systems, Vol. 16, No. 6, December 2007), a micromachined dual-backplate capacitive microphone for aeroacoustic measurements is provided, in which three poly silicon layers are stacked as a single diaphragm and dual backplates, respectively, with the diaphragm sandwiched between the dual backplates. However, this structure cannot be CMOS compatible because of the three low stress poly silicon layers. Also, the three poly silicon plates of the microphone have slightly different radii, rendering the capacitances of the differential condensers a less simple relation.
Patent application International Publication Number WO 2007/089505 disclosed a differential microphone with a rotational diaphragm center-hinged, so that the diaphragm can rock back and forth around the hinge in response to an acoustic wave, thus forming two differential condensers. The microphone is characterized in that it obviates the need for creating a backside chamber, however, the rock of the diaphragm depends on sound wave direction, and there is no perforated backplate for reduction of air damping. Also, such structure may not be proper to be integrated with other CMOS conditioning circuits.
Patent application Publication Number US 2008/0089536 showed a microphone microchip device, in which an additional matching capacitor is offered in order to differentiate the capacitance of the microphone condenser, and a differential receiver is employed to process the difference between the microphone signal and a substantially fixed voltage.
Therefore, there is a need for a CMOS-compatible monolithic silicon microphone chip with differential sensing to increase the signal-to-noise ratio performance, and a method for manufacturing the same.